Field of the Invention
The invention generally relates to a system for attaching electrical equipment to a metallic support.
The invention can especially relate to a system for attaching electrical equipment, for example an inductive sensor adapted to measure the rotation speed of an engine, called sensor N1, on a metallic support, for example a metallic housing of the engine.
Discussion of the Background
Inductive sensors of type N1 are sensors adapted to detect the rotation speed of a phonic wheel of an engine by detection of the passage of teeth in front of the sensor, generating a variation in the magnetic field which creates alternative current in the sensor. The sensor N1 measures and monitors the rotation speed of an engine, and sends this information to a calculator of a control system by means of metallic lugs connected to said sensor N1.
For this, the sensor N1 usually comprises at least one metallic insert interface, generally one for each pole, each connected to a lug by means of metallic pieces adapted to enable transmission of the signal to the control calculator.
Reference could be made in particular to the sensor 1 of type N1 marketed by the company MEGGITT under reference “N1 Speed Probe” for more information on this insert.
The sensor N1 is generally mounted on a structural piece of the housing opposite the phonic wheel. For this, a metallic screw is inserted into each of the inserts so as to fix the sensor on the housing and ensure electrical transmission to the lugs.
However, since the housing is a metallic support, the sensor N1 must be mounted on the support by being electrically insulated from the latter to be able to transmit the electric signal to the calculator.
An example of such assembly is illustrated in FIG. 1a. As is evident, the usual system for attaching comprises a screw 3′, sleeves with metallic collar 40′ and insulating 30′, an electrically insulating spacer 32′, a metallic washer 50′, a lug 60′ and a nut 6′.
The screw 3′ is inserted into the insert 10′ such that the screw head 4′ stops against the insert 10′. Then, the metallic collared sleeve 40′ and the electrically insulating collared sleeve 30′ are fitted successively onto the screw such that the collars of the sleeves are stopped against the insert.
The metallic support 20′ is positioned against the insulating sleeve 30′, then the spacer 32′ which is electrically insulating and mechanically suppler than the other pieces, of overall cylindrical shape, and the metallic washers 50′ are fitted successively in above the metallic support 20′. Finally, the lug 60′ is arranged against the washer 50′ and a nut 6′ is screwed onto the end of the screw 3′ to ensure mechanical retention of all the pieces.
The cylindrical part of the metallic sleeve 40′ is sufficiently long to take up both the insulating sleeve 30′, the metallic support 20′, the insulating spacer 32′ and the metallic washer 50′ along its cylindrical part in contact with the screw 3′, to enable transmission of the electric signal of the insert 10′ to the lug 60′. Also, during tightening of the nut 6′, the insulating collared sleeve 32′ is compressed by the metallic washer 50′ such that electrical contact is made between the insert 10′, the metallic collared sleeve 40′, the metallic washer 50′ and the lug 60′.
In this assembly, transfer of the electric signal from the insert 10′ to the lug 60′ therefore occurs by way of the metallic sleeve 40′ and the metallic washer 50′. Also, its electrical insulation is based on the use of the insulating sleeve 30′ and the supple insulating spacer 32′.
Yet, this assembly is not adapted to be used in an engine, which when operating can undergo temperatures ranging from −55° C. to +160° C. and powerful vibrations from operation of the engine: these stresses in fact deform the spacer 32′ and the insulating sleeve 30′, causing the appearance of considerable play in the assembly, such that the sensor N1 is no longer held in position relative to the metallic support 20′. Also, polymers usually used for the sleeve 30′ and the insulation spacer 32′, for example polytetrafluoroethylene (PTFE) or polyetheretherketone (PEEK), have a vitreous transition temperature incompatible with the operating temperature of the sensor N1.
Document GB 2 227 764 per se proposes a system for attaching a metallic building attachment onto a metallic support for the purpose of limiting problems of bimetallic corrosion. The attachment is fixed to the support by means of a pin, inserted in through a recess formed in the attachment and in the support. A spacer, formed from insulating material, is also fitted on the pin and interposed between the attachment and the support. The system for attaching further comprises a joint, positioned against the spacer.
This system for attaching is also not adapted to be utilised in an engine, to the extent where the effect of the substantial stresses (vibratory environment and severe thermal) undergone by the spacer and the joint would be the appearance of considerable clearance between the support and the attachment. It appears in fact that despite the use of excellent insulating materials, the nut loosens and the insulating pieces deform in such conditions. But it is evident that the aim of this document is not to propose a system for attaching capable of undergoing such a severe environment, to the extent where normal conditions of use of the attachment are those of the building, specifically ambient temperature and negligible vibrations.